Conventional lowering and lifting in subsea environments using an armored umbilical (lowering/communication cable) is by using a winch with the load rating suitable to the task. When lowering a load to extreme depth such as 10,000 feet, the weight of the armored umbilical in water will often exceed the weight of the payload. In the case of a remotely operated vehicle (ROV), the objective is to make the ROV as near neutrally buoyant for ease of operations with only enough weight to allow it to be lowered to the desired depth. The net weight of the ROV plus a handling cage or top hat will be in the range of 1000 lbs., and the armored umbilical getting it to the bottom can exceed 20,000 lbs. Some ROVs are lowered subsea in a heavy cage and swim out as a neutrally buoyant assembly on a short flexible lead. Some ROVs are lowered below a heavy top hat and are released when at the working depth with a short umbilical from a small reel mounted in the top hat.
The armored cable must have substantial capacity as the ROV plus cage or top hat will weigh 1000 lbs. in water, but may well weigh 30,000 lbs. when being lifted through the air/water interface and onto the deck. The winch system at the surface sees its maximum load condition either when it is being lifted through the air/water interface or when it is at its lowest operational depth. Although the ROV plus Top Hat will be only a smaller load such as 1000 lbs., the steel armored umbilical when fully deployed will represent a major load.
With the requirement for 10,000 feet or more in armored cable under tensions up to 30,000 lbs., the crushing load on the drum and the loading on the end flanges which acts similar to pressure, requiring the winch spool to be relatively heavy and expensive to manufacture. The winch torsional requirements for lifting the ROV system out of the water at the air/water interface mandate a substantial gear box to be provided.
An additional difficulty with the conventional winch arrangement is that the cable must be loaded onto the spool with tensions in the range of 12,000 lbs., or when a 30,000 lb. tension load is imparted the current outer wrap of the umbilical will “knife” into the inner wraps and damage in the cable. In some cases the clients insist that the pre-wrapping is at the full 30,000 lbs. tension for added safety. In addition to general difficulties, when a cable is to be replaced, it means it must be taken to shore to be reloaded with equipment which can hold a back tension of 12,000 lbs. (or 30,000 lbs.) tension as it is being spooled.
Some loads similar to this have been handled by coiled tubing injector heads such as the Beta Coiled Tubing Units manufactured by the Beta Division of Brown Oil Tools in the 1970 time frame (U.S. Pat. No. 4,265,304) and is contemporarily done with traction winches on offshore pipe laying vessels. Characteristically, the friction loading against the cable, coiled tubing, or pipeline is from two opposite directions, tending to squash the cable, coiled tubing, or pipeline to an out of round condition which tends to reduce the service life of the components.
Coiled tubing units have sought to engage the coiled tubing from two sides since the 1960s with the resulting loss in service life of the armored umbilical, coiled tubing, and pipeline. This has not been a detriment to pipe line installation as they are installed one time and left in place. However, coiled tubing and armored umbilicals are characteristically service tools deployed and retrieved repeatedly and the added stress of being deformed reduces their usable service life.